1. Field of the Invention
The present invention relates to a fixing apparatus which is configured to transfer and fix an image on an image carrier to a transfer paper and used in electronics copying machines, laser printers, facsimiles or the like, a fixing member used in the fixing apparatus and an image forming apparatus having the fixing apparatus.
2. Description of Related Art
FIG. 7 illustrates a conventional electro-photographic type-image forming apparatus. The conventional image forming apparatus 220, for example, a copier or laser printer includes an image carrier 201 on which electrostatic latent image is formed, an electrification roller 202 disposed to come in contact with the image cattier 201 and configured to charge it, an exposure mechanism 203 such as laser beam, a development roller 204 to attach toner to the latent image on the image carrier 201, a power pack 205 to apply a DC voltage to the electrification roller 202, a transfer roller 206 to transfer the toner image on the image carrier 201 to a recording medium or transfer paper 207, a cleaning device 208 for cleaning the image carrier 201 after transferring, an electrometer 209 to measure a surface potential of the image carrier 201, and a roller type-heat fixing device 210 including a heat fixing roller 211 and a pressure roller 212.
The electro-photographic type-image forming apparatus 220 is configured to charge a photoconductor layer on the rotating image carrier 201 by the electrification roller 202 uniformly, thereafter, expose the image carrier by the exposure mechanism 203 such as the laser beam and form the electrostatic latent image on the image carrier, form the toner image on the image carrier by developing the electrostatic latent image by the toner, transfer the toner image to the transfer paper 207, and fix the toner image on transfer paper 207 by passing the transfer paper between the fixing roller 211 and the pressure roller 212.
The fixing roller 211 used in the heat fixing device 210 of the image forming apparatus 220 includes a core member formed from a cylinder made of a metal such as aluminum or the like and a surface layer (mold-releasing layer) which is made of fluorine resin and provided on an outer periphery of the core member to prevent the toner from adhering to the core member.
The fixing roller 211 has a heater such as a halogen lamp disposed in the core member along a rotational central axis thereof to heat the fixing roller from inside by radiation heat. By passing the transfer paper 207 between the fixing roller 211 and the pressure roller 212, the toner adhered on the transfer paper 207 is pressed by the fixing roller 211 while softening it by heat from the fixing roller 211 and fixed on the transfer paper.
FIG. 8 illustrates a conventional belt type-fixing apparatus. As shown in FIG. 8, the conventional belt type-fixing apparatus 217 includes a fixing belt 213 movably provided by a heating roller 215 and a fixing roller 214 and a pressure roller 216 disposed to come in contact with the fixing roller 214 through the fixing belt 213.
The belt type-fixing device 217 is configured to pass the transfer paper 207 between the fixing belt 213 heated by the heating roller 215 and the pressure roller 216, press toner adhered on the transfer paper 207 by the pressure roller 216 while softening it by heat of the fixing belt 213 and fix the toner to the transfer paper by pressure of the pressure roller 216.
Here, four colors of red (magenta), blue (cyan), yellow and black are used in a full color copying machine or laser printer. When fixing a color image, because colored toners must be mixed in a melted state, it is necessary to be easy to melt colored toners by making them to low melt point, roll up the plurality of colored toners on a surface of the fixing roller and mix them uniformly in the melted state. Accordingly, a system for heating the colored toners a longer time is required.
Therefore, there has been proposed an apparatus in which a heat source is provided in a drive roller which is an intermediate transfer member and a pressure roller is press-contacted with the intermediate transfer member to form a nip (see, for reference, Japanese Patent Laid-Open H10-63121). The apparatus is configured to fix colored toners heated before the nip to a transfer paper by the nip. In the apparatus, because a secondary transfer from the intermediate transfer member to the transfer paper is achieved by means of fixing of heat, not static electricity, a long heating time for the toners can be set.
However, in the apparatus disclosed in Japanese Patent Laid-Open H10-63121, because the intermediate transfer member is also heated by the same heating time as that of the colored toners, and heated throughout the entire thickness from inside, in the next process, the image carrier is also heated when the intermediate transfer member enters a primary transfer area, accordingly there is a possible that the colored toners are adhered to the image carrier.
To resolve this problem, there has been proposed a fixing apparatus capable of increasing fixing efficiency and image quality and accomplishing miniaturization while preventing an intermediate transfer member from being over-heated by a heating mechanism (see, for reference, Japanese Patent Laid-Open 2004-145260).
In this fixing apparatus, because an image is transferred to a fixing member after the image is transferred from a photo conductor or image carrier to the intermediate transfer member, and finally the image is pressed by a fixing member and a pressure roller and fixed on the transfer paper, a long time to heat toner on the fixing member is required, as a result, it is possible to provide sufficient heat quantity to melt a plurality of colored toners efficiently, accordingly, fix an image having sufficient glaze and less glaze-variation.
In the fixing apparatus, because a low fixing temperature at a nip formed between the fixing member and the pressure roller can be used, it is possible to reduce a starting time and therefore accomplish less energy. In addition, because a nip having a small area is sufficient for a pressing section, it is possible to minimize heat transmission to the transfer paper, therefore to accomplish efficient heating and pressing.
However, in this fixing apparatus, to reduce heat-deterioration of the image carrier, heat transmission from the fixing member having the heating mechanism to the intermediate transfer member contacting with the image carrier, and the pressure roller is required to be limited wherever possible, it is necessary to provide a heat shielding member or heat insulating plate to limit the heat transmission and a cooling roller or the like to draw heat from the intermediate transfer member between a transfer section of the intermediate transfer member and a transfer section from the image carrier.
Furthermore, in this fixing apparatus, when the movement of the transfer paper is stopped by a jam generated when the transfer paper passes the nip formed between the fixing member and the pressure roller, there is a problem that an excessive heat loading is applied to one portion of the fixing member. Because a cooling mechanism provided in the fixing apparatus is designed under heat incoming-loss in a series of fixing cycles, when the feeding of the transfer paper is restarted, or when the transfer paper which remains heated is fed, unexpected heat is transmitted to the fixing member so that the original aim of the fixing member is not achieved, excessive heat is transmitted to the intermediate transfer member, further one portion the excessive heat is transmitted to the image carrier. Consequently, there is a problem that deterioration of the surface of the image carrier is promoted. Moreover, there is a problem that the over heat causes surface-deterioration of the fixing member itself or layer-separation of the fixing member if the fixing member includes a plurality of surface layers.